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Saturday, January 10, 2015

A Bussard Ramjet, one of many possible methods that could serve as propulsion for a starship.

Interstellar space travel is manned or unmanned travel between stars. Interstellar travel is much more difficult than interplanetary travel: the distances between the planets in the Solar System are typically measured in standard astronomical units (AU)—whereas the distances between stars are typically hundreds of thousands of AU, and usually expressed in light-years. Because of the vastness of those distances, interstellar travel would require either great speed (some percentage of the speed of light) or huge travel time (lasting from years to millennia).

The required speeds for interstellar travel in a human lifespan are far beyond what current methods of spacecraft propulsion
can provide. The energy required to propel a spacecraft to these
speeds, regardless of the propulsion system used, is enormous by today's
standards of energy production.
At these speeds, collisions by the spacecraft with interstellar dust
and gas can produce very dangerous effects both to any passengers and
the spacecraft itself.

A number of widely differing strategies have been proposed to deal
with these problems, ranging from giant arks that would carry entire
societies and ecosystems very slowly, to microscopic space probes. Many
different propulsion systems have been proposed to give spacecraft the
required speeds: these range from different forms of nuclear propulsion,
to beamed energy methods that would require megascale engineering projects, to methods based on speculative physics.

For both unmanned and manned interstellar travel, considerable
technological and economic challenges would need to be met. Even the
most optimistic views about interstellar travel are that it might happen
decades in the future; the more common view is that it is a century or
more away.

Challenges

Interstellar distances

The basic challenge facing interstellar travel is the immense distances between the stars.
Astronomical distances are measured using different units of length,
depending on the scale of the distances involved. Between the planets in
the Solar System they are often measured in astronomical units (AU), defined as the average distance between the Sun and Earth, some 150 million kilometers (93 million miles). Venus, the closest other planet to Earth is (at closest approach) 0.28 AU away. Neptune, the furthest planet from the Sun, is 29.8 AU away. Voyager 1, the furthest man-made object from Earth, is 129.2 AU away.

The closest known star Proxima Centauri, however, is some 268,332 AU away, or 9000 times further away than even the furthest planet in the Solar System.

Object

A.U.

light time

The Moon

0.0026

1.3 seconds

Venus (nearest planet)

0.28

2.41 minutes

Neptune (furthest planet)

29.8

4.1 hours

Voyager 1

129.2

17.9 hours

Proxima Centauri (nearest star)

268,332

4.24 years

Because of this, distances between stars are usually expressed in light-years, defined as the distance that a ray of light
travels in a year. Light in a vacuum travels around 300,000 kilometers
(186,000 miles) per second, so this is some 9.46 trillion kilometers
(5.87 trillion miles) or 63,241 AU. Proxima Centauri is 4.243
light-years away.

Another way of understanding the vastness of interstellar distances
is by scaling: one of the closest stars to the sun, Alpha Centauri A (a
Sun-like star), can be pictured by scaling down the Earth–Sun distance to one meter (~3.3 ft). On this scale, the distance to Alpha Centauri A would be 271 kilometers (169 miles).
The fastest outward-bound spacecraft yet sent, Voyager 1,
has covered 1/600th of a light-year in 30 years and is currently moving
at 1/18,000th the speed of light. At this rate, a journey to Proxima
Centauri would take 80,000 years.[1]

Some combination of great speed and long travel time are required.
The time required by propulsion methods based on currently known
physical principles would require years to millennia.

Required energy

A significant factor contributing to the difficulty is the energy
that must be supplied to obtain a reasonable travel time. A lower bound
for the required energy is the kinetic energy K = ½ mv2 where m is the final mass. If deceleration
on arrival is desired and cannot be achieved by any means other than
the engines of the ship, then the required energy is significantly
increased.

The velocity for a manned round trip of a few decades to even the
nearest star is several thousand times greater than those of present
space vehicles. This means that due to the v2 term in the
kinetic energy formula, millions of times as much energy is required.
Accelerating one ton to one-tenth of the speed of light requires at
least 450 PJ or 4.5 ×1017 J
or 125 billion kWh, without factoring in efficiency of the propulsion
mechanism. This energy has to be generated on-board from stored fuel,
harvested from the interstellar medium, or projected over immense
distances.

Manned missions

The mass of any craft capable of carrying humans would inevitably be substantially larger than that necessary for an unmanned interstellar probe. For instance, the first space probe, Sputnik 1, had a payload of 83.6 kg, whereas the first spacecraft carrying a living passenger (the dog Laika), Sputnik 2,
had a payload six times that at 508.3 kg. This underestimates the
difference in the case of interstellar missions, given the vastly
greater travel times involved and the resulting necessity of a closed-cyclelife support system.
As technology continues to advance, combined with the aggregate risks
and support requirements of manned interstellar travel, the first
interstellar missions are unlikely to carry life forms.

A manned craft will require more time to reach its top speed as humans have limited tolerance to acceleration.

Interstellar medium

A major issue with traveling at extremely high speeds is that interstellar dust and gas
may cause considerable damage to the craft, due to the high relative
speeds and large kinetic energies involved.
Various shielding methods to
mitigate this problem have been proposed.[2]
Larger objects (such as macroscopic dust grains) are far less common,
but would be much more destructive. The risks of impacting such objects,
and methods of mitigating these risks, have been discussed in the
literature, but many unknowns remain.[3]

Wait calculation

It has been argued that an interstellar mission that cannot be
completed within 50 years should not be started at all. Instead,
assuming that a civilization is still on an increasing curve of
propulsion system velocity, not yet having reached the limit, the
resources should be invested in designing a better propulsion system.
This is because a slow spacecraft would probably be passed by another
mission sent later with more-advanced propulsion (the incessant
obsolescence postulate).[7]
On the other hand, Andrew Kennedy has shown that if one calculates the
journey time to a given destination as the rate of travel speed derived
from growth (even exponential growth) increases, there is a clear
minimum in the total time to that destination from now (see wait calculation).[8]
Voyages undertaken before the minimum will be overtaken by those who
leave at the minimum, whereas those who leave after the minimum will
never overtake those who left at the minimum.

One argument against the stance of delaying a start until reaching
fast propulsion system velocity is that the various other non-technical
problems that are specific to long-distance travel at considerably
higher speed (such as interstellar particle impact, possible dramatic
shortening of average human life span during extended space residence,
etc.) may remain obstacles that take much longer time to resolve than
the propulsion issue alone, assuming that they can even be solved
eventually at all. A case can therefore be made for starting a mission
without delay, based on the concept of an achievable and dedicated but
relatively slow interstellar mission using the current technological
state-of-the-art and at relatively low cost, rather than banking on
being able to solve all problems associated with a faster mission
without having a reliable time frame for achievability of such.

Communications

The round-trip delay time
is the minimum time between an observation by the probe and the moment
the probe can receive instructions from Earth reacting to the
observation. Given that information can travel no faster than the speed of light, this is for the Voyager 1
about 36 hours, and near Proxima Centauri it would be 8 years. Faster
reaction would have to be programmed to be carried out automatically. Of
course, in the case of a manned flight the crew can respond immediately
to their observations. However, the round-trip delay time makes them
not only extremely distant from, but, in terms of communication,
also extremely isolated from Earth (analogous to how past long distance
explorers were similarly isolated before the invention of the electrical telegraph).

Interstellar communication is still problematic – even if a probe
could reach the nearest star, its ability to communicate back to Earth
would be difficult given the extreme distance. See Interstellar communication.

Prime targets for interstellar travel

There are 59 known stellar systems within 20 light years of the Sun,
containing 81 visible stars. The following could be considered prime
targets for interstellar missions:[9]

A system with at least six planets. A record-breaking three of these
planets are super-Earths lying in the zone around the star where liquid
water could exist, making them possible candidates for the presence of
life.[12]

At least one planet, and of a suitable age to have evolved primitive life [13]

Existing and near-term astronomical technology is capable of finding
planetary systems around these objects, increasing their potential for
exploration.

Proposed methods

Slow, uncrewed probes

Slow interstellar missions based on current and near-future
propulsion technologies are associated with trip times starting from
about one hundred years to thousands of years. These missions consist of
sending a robotic probe to a nearby star for exploration, similar to
interplanetary probes such as used in the voyager program.
By taking along no crew, the cost and complexity of the mission is
significantly reduced although technology lifetime is still a
significant issue next to obtaining a reasonable speed of travel.
Proposed concepts include Project Daedalus, Project Icarus and Project Longshot.

Fast, uncrewed probes

Nanoprobes

Near-lightspeed nanospacecraft might be possible within the near
future built on existing microchip technology with a newly developed
nanoscale thruster. Researchers at the University of Michigan are
developing thrusters that use nanoparticles as propellant. Their
technology is called “nanoparticle field extraction thruster”, or nanoFET. These devices act like small particle accelerators shooting conductive nanoparticles out into space.[14]

Michio Kaku,
a theoretical physicist, has suggested that clouds of "smart dust" be
sent to the stars, which may become possible with advances in nanotechnology.
Kaku also notes that a large amount of nanoprobes would need to be sent
due to the vulnerability of very small probes to be easily deflected by
magnetic fields, micrometeorites and other dangers to ensure the
chances that at least one nanoprobe will survive the journey and reach
the destination.[15]

Given the light weight of these probes, it would take much less
energy to accelerate them. With on board solar cells they could
continually accelerate using solar power. One can envision a day when a
fleet of millions or even billions of these particles swarm to distant
stars at nearly the speed of light and relay signals back to Earth
through a vast interstellar communication network.

Slow, manned missions

In crewed missions, the duration of a slow interstellar journey
presents a major obstacle and existing concepts deal with this problem
in different ways.[16] They can be distinguished by the "state" in which humans are transported on-board of the spacecraft.

Generation ships

A generation ship (or world ship) is a type of interstellar ark
in which the crew that arrives at the destination is descended from
those who started the journey. Generation ships are not currently
feasible because of the difficulty of constructing a ship of the
enormous required scale and the great biological and sociological
problems that life aboard such a ship raises.[17][18][19][20]

Extended human lifespan

A variant on this possibility is based on the development of substantial human life extension, such as the "Strategies for Engineered Negligible Senescence" proposed by Dr. Aubrey de Grey.
If a ship crew had lifespans of some thousands of years, or had
artificial bodies, they could traverse interstellar distances without
the need to replace the crew in generations. The psychological effects
of such an extended period of travel would potentially still pose a
problem.

Mind uploading

A more speculative method of transporting humans to the stars is by using mind uploading or also called brain emulation.[23][24]Frank J. Tipler speculates about the colonization of the universe by starships transporting uploaded humans.[25]
Hein presents a range of concepts how such missions could be conducted,
using more or less speculative technologies, for example self-replicating machines, wormholes, and teleportation.[23][26] One of the major challenges besides mind uploading itself are the means for downloading the uploads into physical entities, which can be biological or artficial or both.

Island hopping through interstellar space

Interstellar space is not completely empty; it contains trillions of icy bodies ranging from small asteroids (Oort cloud) to possible rogue planets.
There may be ways to take advantage of these resources for a good part
of an interstellar trip, slowly hopping from body to body or setting up
waystations along the way.[27]

Fast missions

If a spaceship could average 10 percent of light speed (and
decelerate at the destination, for manned missions), this would be
enough to reach Proxima Centauri
in forty years. Several propulsion concepts are proposed that might be
eventually developed to accomplish this (see section below on propulsion
methods), but none of them are ready for near-term (few decades)
development at acceptable cost.[citation needed]

Time dilation

Assuming one cannot travel faster than light, one might conclude that
a human can never make a round-trip further from Earth than 40 light
years if the traveler is active between the ages of 20 and 60. In this
example a traveler would never be able to reach more than the very few
star systems that exist within the limit of 10–20 light years from
Earth. This, however, fails to take into account time dilation.
Clocks aboard an interstellar ship would run slower than Earth clocks,
so if a ship's engines were powerful enough the ship could reach mostly
anywhere in the galaxy and return to Earth within 40 years ship-time.
Upon return, there would be a difference between the time elapsed on the
astronaut's ship and the time elapsed on Earth. If a spaceship travels
to a star 32 light-years away and initially accelerates at a constant
1.03g (i.e. 10.1 m/s2) for 1.32 years (ship time) then stops
its engines and coasts for the next 17.3 years (ship time) at a constant
speed then decelerates again for 1.32 ship-years and comes to a stop at
the destination. After a short visit the astronaut returns to Earth the
same way.
After the full round-trip, the clocks on board the ship show that 40
years have passed, but according to those on Earth, the ship comes back
76 years after launch.

From the viewpoint of the astronaut, on-board clocks seem to be
running normally. The star ahead seems to be approaching at a speed of
0.87 lightyears per ship-year. The universe would appear contracted
along the direction of travel to half the size it had when the ship was
at rest; the distance between that star and the Sun would seem to be 16
light years as measured by the astronaut.

At higher speeds, the time onboard will run even slower, so the astronaut could travel to the center of the Milky Way
(30 kly from Earth) and back in 40 years ship-time. But the speed
according to Earth clocks will always be less than 1 lightyear per Earth
year, so, when back home, the astronaut will find that 60 thousand
years will have passed on Earth.[citation needed]

Constant acceleration

This plot shows a ship capable of 1-gee (10 m/s2 or about 1.0 ly/y2) "felt" or proper-acceleration[28] can go far, except for the problem of accelerating on-board propellant.

Regardless of how it is achieved, if a propulsion system can produce
acceleration continuously from departure to destination, then this will
be the fastest method of travel. If the propulsion system drives the
ship faster and faster for the first half of the journey, then turns
around and brakes the craft so that it arrives at the destination at a
standstill, this is a constant acceleration journey. If this was
performed at nearly 1g, this would have the added advantage of producing
artificial "gravity". This is, however, largely unfeasible with current
technology because of the difficulty in maintaining acceleration the
closer one gets to the speed of light. This is illustrated by the
definition of force: F=dp/dt. This is also a part of Newton's second law
of motion.[29]

From the planetary observer perspective the ship will appear to
steadily accelerate but more slowly as it approaches the speed of light.
The ship will be close to the speed of light after about a year of
accelerating and remain at that speed until it brakes for the end of the
journey.

From the ship perspective there will be no top limit on speed – the
ship keeps going faster and faster the whole first half. This happens
because the ship's time sense slows down – relative to the planetary
observer – the more it approaches the speed of light.

The result is an impressively fast journey if you are in the ship.

By transmission

If physical entities could be transmitted as information and
reconstructed at a destination, travel at nearly the speed of light
would be possible, which for the "travelers" would be instantaneous.
However, sending an atom-by-atom description of (say) a human body would
be a daunting task. Extracting and sending only a computer brain simulation
is a significant part of that problem. "Journey" time would be the
light-travel time plus the time needed to encode, send and reconstruct
the whole transmission.[30]

Propulsion

Rocket concepts

All rocket concepts are limited by the rocket equation, which sets the characteristic velocity available as a function of exhaust velocity and mass ratio, the ratio of initial (M0, including fuel) to final (M1, fuel depleted) mass.

Very high specific power, the ratio of thrust to total vehicle mass, is required to reach interstellar targets within sub-century time-frames.[31] Some heat transfer is inevitable and a tremendous heating load must be adequately handled.

Thus, for interstellar rocket concepts of all technologies, a key
engineering problem (seldom explicitly discussed) is limiting the heat
transfer from the exhaust stream back into the vehicle.[32]

Nuclear fission powered

Fission-electric

Nuclear-electric or plasma engines, operating for long periods at low
thrust and powered by fission reactors, have the potential to reach
speeds much greater than chemically powered vehicles or nuclear-thermal
rockets. Such vehicles probably have the potential to power Solar System
exploration with reasonable trip times within the current century.
Because of their low-thrust propulsion, they would be limited to
off-planet, deep-space operation. Electrically powered spacecraft propulsion powered by a portable power-source, say a nuclear reactor, producing only small accelerations, would take centuries to reach for example 15% of the velocity of light, thus unsuitable for interstellar flight during a single human lifetime.[33]

Fission-fragment

Fission-fragment rockets use nuclear fission
to create high-speed jets of fission fragments, which are ejected at
speeds of up to 12,000 km/s. With fission, the energy output is
approximately 0.1% of the total mass-energy of the reactor fuel and
limits the effective exhaust velocity to about 5% of the velocity of
light. For maximum velocity, the reaction mass should optimally consist
of fission products, the "ash" of the primary energy source, in order
that no extra reaction mass need be book-kept in the mass ratio. This is
known as a fission-fragment rocket. thermal-propulsion engines such as NERVA
produce sufficient thrust, but can only achieve relatively low-velocity
exhaust jets, so to accelerate to the desired speed would require an
enormous amount of fuel.

Nuclear pulse

Based on work in the late 1950s to the early 1960s, it has been technically possible to build spaceships with nuclear pulse propulsion engines, i.e. driven by a series of nuclear explosions. This propulsion system contains the prospect of very high specific impulse (space travel's equivalent of fuel economy) and high specific power.[34]Project Orion team member, Freeman Dyson, proposed in 1968 an interstellar spacecraft using nuclear pulse propulsion that used pure deuterium fusion detonations with a very high fuel-burnup fraction. He computed an exhaust velocity of 15,000 km/s and a 100,000-tonne space vehicle able to achieve a 20,000 km/s delta-v allowing a flight-time to Alpha Centauri of 130 years.[35]
Later studies indicate that the top cruise velocity that can
theoretically be achieved by a Teller-Ulam thermonuclear unit powered
Orion starship, assuming no fuel is saved for slowing back down, is
about 8% to 10% of the speed of light (0.08-0.1c).[36]
An atomic (fission) Orion can achieve perhaps 3%-5% of the speed of
light. A nuclear pulse drive starship powered by Fusion-antimatter
catalyzed nuclear pulse propulsion units would be similarly in the 10%
range and pure Matter-antimatter annihilation rockets would be
theoretically capable of obtaining a velocity between 50% to 80% of the
speed of light. In each case saving fuel for slowing down halves the
max. speed. The concept of using a magnetic sail to decelerate the
spacecraft as it approaches its destination has been discussed as an
alternative to using propellant, this would allow the ship to travel
near the maximum theoretical velocity.[37] Alternative designs utilizing similar principles include Project Longshot, Project Daedalus, and Mini-Mag Orion.
The principle of external nuclear pulse propulsion to maximize
survivable power has remained common among serious concepts for
interstellar flight without external power beaming and for very
high-performance interplanetary flight.

A current impediment to the development of any nuclear-explosion-powered spacecraft is the 1963 Partial Test Ban Treaty,
which includes a prohibition on the detonation of any nuclear devices
(even non-weapon based) in outer space. This treaty would therefore need
to be renegotiated, although a project on the scale of an interstellar
mission using currently foreseeable technology would probably require
international cooperation on at least the scale of the International Space Station.

Nuclear fusion rockets

Fusion rocket starships, powered by nuclear fusion
reactions, should conceivably be able to reach speeds of the order of
10% of that of light, based on energy considerations alone. In theory, a
large number of stages could push a vehicle arbitrarily close to the
speed of light.[39] These would "burn" such light element fuels as deuterium, tritium, 3He, 11B, and 7Li.
Because fusion yields about 0.3–0.9% of the mass of the nuclear fuel as
released energy, it is energetically more favorable than fission, which
releases <0 .1="" 4="" a="" achievable="" although="" and="" are="" as="" available="" be="" best="" c.="" centuries.="" concepts="" correspondingly="" decades="" difficulties="" easily="" energetically="" energy="" engineering="" exhaust="" fission="" for="" fraction="" fuel="" fusion="" high-energy="" higher="" however="" human="" intractable="" involve="" large="" lifetime="" long="" loss.="" mass-energy.="" massive="" maximum="" may="" most="" nearest-term="" nearest="" neutrons="" of="" offer="" or="" out="" p="" potentially="" prospects="" reactions="" release="" s="" seem="" significant="" source="" stars="" still="" technological="" than="" the="" their="" these="" they="" thus="" to="" travel="" turn="" typically="" velocities="" which="" within="">
Early studies include Project Daedalus, performed by the British Interplanetary Society in 1973–1978, and Project Longshot, a student project sponsored by NASA and the US Naval Academy, completed in 1988. Another fairly detailed vehicle system, "Discovery II",[40] designed and optimized for crewed Solar System exploration, based on the D3He reaction but using hydrogen as reaction mass, has been described by a team from NASA's Glenn Research Center. It achieves characteristic velocities of >300 km/s with an acceleration of ~1.7•10−3g,
with a ship initial mass of ~1700 metric tons, and payload fraction
above 10%. Although these are still far short of the requirements for
interstellar travel on human timescales, the study seems to represent a
reasonable benchmark towards what may be approachable within several
decades, which is not impossibly beyond the current state-of-the-art.
Based on the concept's 2.2% burnup fraction it could achieve a pure fusion product exhaust velocity of ~3,000 km/s.

Antimatter rockets

An antimatter rocket
would have a far higher energy density and specific impulse than any
other proposed class of rocket. If energy resources and efficient
production methods are found to make antimatter
in the quantities required and store it safely, it would be
theoretically possible to reach speeds approaching that of light. Then
relativistic time dilation
would become more noticeable, thus making time pass at a slower rate
for the travelers as perceived by an outside observer, reducing the trip
time experienced by human travelers.

Supposing the production and storage of antimatter should become
practical, two further problems would present and need to be solved.
First, in the annihilation of antimatter, much of the energy is lost in
very penetrating high-energy gamma radiation, and especially also in
neutrinos, so that substantially less than mc2 would
actually be available if the antimatter were simply allowed to
annihilate into radiations thermally. Even so, the energy available for
propulsion would probably be substantially higher than the ~1% of mc2 yield of nuclear fusion, the next-best rival candidate.

Second, once again heat transfer from exhaust to vehicle seems likely
to deposit enormous wasted energy into the ship, considering the large
fraction of the energy that goes into penetrating gamma rays. Even
assuming biological shielding were provided to protect the passengers,
some of the energy would inevitably heat the vehicle, and may thereby
prove limiting. This requires consideration for serious proposals if
useful accelerations are to be achieved, because the energies involved
(e.g. for 0.1g ship acceleration, approaching 0.3 trillion watts per ton of ship mass) are very large.

Rockets with an external energy source

Rockets deriving their power from external sources, such as a laser,
could bypass the ordinary rocket equation, potentially reducing the
mass of the ship greatly and allowing much higher travel speeds. Geoffrey A. Landis has proposed for an interstellar probe, with energy supplied by an external laser from a base station powering an Ion thruster.[41]

Non-rocket concepts

A problem with all traditional rocket propulsion methods is that the
spacecraft would need to carry its fuel with it, thus making it very
massive, in accordance with the rocket equation. Some concepts attempt to escape from this problem ([42]):

Interstellar ramjets

In 1960, Robert W. Bussard proposed the Bussard ramjet, a fusion rocket in which a huge scoop would collect the diffuse hydrogen in interstellar space, "burn" it on the fly using a proton–proton fusion
reaction, and expel it out of the back. Later calculations with more
accurate estimates suggest that the thrust generated would be less than
the drag caused by any conceivable scoop design. Yet the idea is
attractive because the fuel would be collected en route (commensurate with the concept of energy harvesting), so the craft could theoretically accelerate to near the speed of light.

Beamed propulsion

A light sail or magnetic sail powered by a massive laser
or particle accelerator in the home star system could potentially reach
even greater speeds than rocket- or pulse propulsion methods, because
it would not need to carry its own reaction mass and therefore would only need to accelerate the craft's payload. Robert L. Forward
proposed a means for decelerating an interstellar light sail in the
destination star system without requiring a laser array to be present in
that system. In this scheme, a smaller secondary sail is deployed to
the rear of the spacecraft, whereas the large primary sail is detached
from the craft to keep moving forward on its own. Light is reflected
from the large primary sail to the secondary sail, which is used to
decelerate the secondary sail and the spacecraft payload.[44]

A magnetic sail
could also decelerate at its destination without depending on carried
fuel or a driving beam in the destination system, by interacting with
the plasma found in the solar wind of the destination star and the
interstellar medium.[45][46]

The following table lists some example concepts using beamed laser propulsion as proposed by the physicist Robert L. Forward:[47]

Pre-accelerated fuel

Achieving start-stop interstellar trip times of less than a human
lifetime require mass-ratios of between 1,000 and 1,000,000, even for
the nearer stars. This could be achieved by multi-staged vehicles on a
vast scale.[39] Alternatively large linear accelerators could propel fuel to fission propelled space-vehicles, avoiding the limitations of the Rocket equation.[48]

Speculative methods

Quark matter

Scientist T. Marshall Eubanks thinks that nuggets of condensed quark matter may exist at the centers of some asteroids, created during the Big Bang and each nugget with a mass of 1010 to 1011 kg.[49]
If so these could be an enormous source of energy, as the nuggets could
be used to generate huge quantities of antimatter—about a million
tonnes of antimatter per nugget. This would be enough to propel a
spacecraft close to the speed of light.[50]

π0 decays rapidly to two photons, and the positron
annihilates with an electron to give two more photons. As a result, a
hydrogen atom turns into four photons and only the problem of a mirror
remains unresolved.

A magnetic monopole engine could also work on a once-through scheme such as the Bussard ramjet (see below).

Faster-than-light travel

Scientists and authors have postulated a number of ways by which it
might be possible to surpass the speed of light. Even the most
serious-minded of these are speculative.

It is also debated whether this is possible, in part, because of causality concerns, because in essence travel faster than light is equivalent to going back in time. Proposed mechanisms for faster-than-light travel within the theory of general relativity require the existence of exotic matter.

Alcubierre drive

General relativity may permit the travel of an object faster than light in curved spacetime.[55] One could imagine exploiting the curvature to take a "shortcut" from one point to another. This is one form of the warp drive concept.

In physics, the Alcubierre drive
is based on an argument that the curvature could take the form of a
wave in which a spaceship might be carried in a "bubble". Space would be
collapsing at one end of the bubble and expanding at the other end. The
motion of the wave would carry a spaceship from one space point to
another in less time than light would take through unwarped space.
Nevertheless, the spaceship would not be moving faster than light within
the bubble. This concept would require the spaceship to incorporate a
region of exotic matter, or "negative mass".

Artificial gravity control

Scientist Lance Williams thinks that gravity can be controlled artificially through electromagnetic control.[56]

Wormholes

Wormholes
are conjectural distortions in spacetime that theorists postulate could
connect two arbitrary points in the universe, across an Einstein–Rosen Bridge.
It is not known whether wormholes are possible in practice. Although
there are solutions to the Einstein equation of general relativity that
allow for wormholes, all of the currently known solutions involve some
assumption, for example the existence of negative mass, which may be unphysical.[57] However, Cramer et al. argue that such wormholes might have been created in the early universe, stabilized by cosmic string.[58] The general theory of wormholes is discussed by Visser in the book Lorentzian Wormholes.[59]

Designs and studies

Enzmann starship

The Enzmann starship, as detailed by G. Harry Stine in the October 1973 issue of Analog, was a design for a future starship, based on the ideas of Dr. Robert Duncan-Enzmann.[60] The spacecraft itself as proposed used a 12,000,000 ton ball of frozen deuterium to power 12–24 thermonuclear pulse propulsion units.[60] Twice as long as the Empire State Building and assembled in-orbit, the spacecraft was part of a larger project preceded by interstellar probes and telescopic observation of target star systems.[60][61]

Project Hyperion

NASA research

NASA
has been researching interstellar travel since its formation,
translating important foreign language papers and conducting early
studies on applying fusion propulsion, in the 1960s, and laser
propulsion, in the 1970s, to interstellar travel.

Geoffrey A. Landis of NASA's Glenn Research Center
states that a laser-powered interstellar sail ship could possibly be
launched within 50 years, using new methods of space travel. "I think
that ultimately we're going to do it, it's just a question of when and
who," Landis said in an interview. Rockets are too slow to send humans
on interstellar missions. Instead, he envisions interstellar craft with
extensive sails, propelled by laser light to about one-tenth the speed
of light. It would take such a ship about 43 years to reach Alpha
Centauri, if it passed through the system. Slowing down to stop at Alpha
Centauri could increase the trip to 100 years,[65]
whereas a journey without slowing down raises the issue of making
sufficiently accurate and useful observations and measurements during a
fly-by.

100 Year Starship study

The 100 Year Starship
(100YSS) is the name of the overall effort that will, over the next
century, work toward achieving interstellar travel. The effort will also
go by the moniker 100YSS. The 100 Year Starship study is the name of a
one year project to assess the attributes of and lay the groundwork for
an organization that can carry forward the 100 Year Starship vision.

Non-profit organisations

A few organisations dedicated to interstellar propulsion research and
advocacy for the case exist worldwide. These are still in their
infancy, but are already backed up by a membership of a wide variety of
scientists, students and professionals.

Skepticism

The energy requirements make interstellar travel very difficult. It
has been reported that at the 2008 Joint Propulsion Conference, multiple
experts opined that it was improbable that humans would ever explore
beyond the Solar System.[75]
Brice N. Cassenti, an associate professor with the Department of
Engineering and Science at Rensselaer Polytechnic Institute, stated at
least the total energy output of the entire world [in a given year]
would be required to send a probe to the nearest star.[75]
0>

While the observation of objects in space, known as astronomy, predates reliable recorded history, it was the development of large and relatively efficient rockets
during the early 20th century that allowed physical space exploration
to become a reality. Common rationales for exploring space include
advancing scientific research, uniting different nations, ensuring the
future survival of humanity and developing military and strategic
advantages against other countries.

With the substantial completion of the ISS[1] following STS-133 in March 2011, plans for space exploration by the USA remain in flux. Constellation, a Bush Administration program for a return to the Moon by 2020[2] was judged inadequately funded and unrealistic by an expert review panel reporting in 2009.[3]
The Obama Administration proposed a revision of Constellation in 2010
to focus on the development of the capability for crewed missions beyond
low earth orbit (LEO), envisioning extending the operation of the ISS beyond 2020, transferring the development of launch vehicles for human crews from NASA to the private sector, and developing technology to enable missions to beyond LEO, such as Earth/Moon L1, the Moon, Earth/Sun L2, near-earth asteroids, and Phobos or Mars orbit.[4]

History of exploration in the 20th century

Most orbital flight actually takes place in upper layers of the atmosphere, especially in the thermosphere (not to scale)

Timeline of Solar System exploration.

In July 1950 the first Bumper rocket is launched from Cape Canaveral, Florida. The Bumper was a two-stage rocket consisting of a Post-War V-2 topped by a WAC Corporal
rocket. It could reach then-record altitudes of almost 400 km. Launched
by General Electric Company, this Bumper was used primarily for testing
rocket systems and for research on the upper atmosphere. They carried
small payloads that allowed them to measure attributes including air
temperature and cosmic ray impacts.

The first steps of putting a man-made object into space were taken by German scientists during World War II while testing the V-2 rocket, which became the first man-made object in space on 3 October 1942 with the launching of the A-4. After the war, the U.S. used German scientists
and their captured rockets in programs for both military and civilian
research. The first scientific exploration from space was the cosmic
radiation experiment launched by the U.S. on a V-2 rocket on 10 May
1946.[5] The first images of Earth taken from space followed the same year[6][7] while the first animal experiment
saw fruit flies lifted into space in 1947, both also on modified V-2s
launched by Americans. Starting in 1947, the Soviets, also with the help
of German teams, launched sub-orbital V-2 rockets and their own
variant, the R-1, including radiation and animal experiments on some flights. These suborbital experiments only allowed a very short time in space which limited their usefulness.

The first successful orbital launch was of the Soviet unmanned Sputnik 1 ("Satellite 1")
mission on 4 October 1957. The satellite weighed about 83 kg (183 lb),
and is believed to have orbited Earth at a height of about 250 km
(160 mi). It had two radio transmitters (20 and 40 MHz), which emitted
"beeps" that could be heard by radios around the globe. Analysis of the
radio signals was used to gather information about the electron density
of the ionosphere, while temperature and pressure data was encoded in
the duration of radio beeps. The results indicated that the satellite
was not punctured by a meteoroid. Sputnik 1 was launched by an R-7 rocket. It burned up upon re-entry on 3 January 1958.

This success led to an escalation of the American space program, which unsuccessfully attempted to launch a Vanguard satellite into orbit two months later. On 31 January 1958, the U.S. successfully orbited Explorer 1 on a Juno rocket. In the meantime, the Soviet dog Laika became the first animal in orbit on 3 November 1957.

First human flights

The first successful human spaceflight was Vostok 1 ("East 1"), carrying 27 year old Russian cosmonautYuri Gagarin
on 12 April 1961. The spacecraft completed one orbit around the globe,
lasting about 1 hour and 48 minutes. Gagarin's flight resonated around
the world; it was a demonstration of the advanced Soviet space program and it opened an entirely new era in space exploration: human spaceflight.

China first launched a person into space 42 years after the launch of Vostok 1, on 15 October 2003, with the flight of Yang Liwei aboard the Shenzhou 5 (Spaceboat 5) spacecraft.

First planetary explorations

The first artificial object to reach another celestial body was Luna 2 in 1959.[8] The first automatic landing on another celestial body was performed by Luna 9[9] in 1966. Luna 10 became the first artificial satellite of the Moon.[10]

The first manned landing on another celestial body was performed by Apollo 11 in its lunar landing on 20 July 1969.

The first interplanetary surface mission to return at least limited surface data from another planet was the 1970 landing of Venera 7 on Venus which returned data to earth for 23 minutes. In 1971 the Mars 3
mission achieved the first soft landing on Mars returning data for
almost 20 seconds. Later much longer duration surface missions were
achieved, including over 6 years of Mars surface operation by Viking 1 from 1975 to 1982 and over 2 hours of transmission from the surface of Venus by Venera 13 in 1982, the longest ever Soviet planetary surface mission.

Key people in early space exploration

The dream of stepping into the outer reaches of the Earth's atmosphere was driven by the fiction of Jules Verne[11][12][13] and H.G.Wells,[14] and rocket technology was developed to try to realise this vision.

The German V-2
was the first rocket to travel into space, overcoming the problems of
thrust and material failure. During the final days of World War II this
technology was obtained by both the Americans and Soviets as were its
designers. The initial driving force for further development of the
technology was a weapons race for intercontinental ballistic missiles (ICBMs) to be used as long-range carriers for fast nuclear weapon delivery, but in 1961 when USSR launched the first man into space, the U.S. declared itself to be in a "Space Race" with the Soviets.

Wernher von Braun was the lead rocket engineer for Nazi Germany's World War II V-2
rocket project. In the last days of the war he led a caravan of workers
in the German rocket program to the American lines, where they
surrendered and were brought to the USA to work on U.S. rocket
development ("Operation Paperclip"). He acquired American citizenship and led the team that developed and launched Explorer 1, the first American satellite. Von Braun later led the team at NASA's Marshall Space Flight Center which developed the Saturn V moon rocket.

Initially the race for space was often led by Sergei Korolyov, whose legacy includes both the R7 and Soyuz—which
remain in service to this day. Korolev was the mastermind behind the
first satellite, first man (and first woman) in orbit and first
spacewalk. Until his death his identity was a closely guarded state
secret; not even his mother knew that he was responsible for creating
the Soviet space program.Kerim Kerimov was one of the founders of the Soviet space program and was one of the lead architects behind the first human spaceflight (Vostok 1)
alongside Sergey Korolyov. After Korolyov's death in 1966, Kerimov
became the lead scientist of the Soviet space program and was
responsible for the launch of the first space stations from 1971 to 1991, including the Salyut and Mir series, and their precursors in 1967, the Cosmos 186 and Cosmos 188.[15][16]

Other key people

Valentin Glushko
held the role of Chief Engine Designer for USSR. Glushko designed many
of the engines used on the early Soviet rockets, but was constantly at
odds with Korolyov.

Vasily Mishin
was Chief Designer working under Sergey Korolyov and one of first
Soviets to inspect the captured German V-2 design. Following the death
of Sergei Korolev, Mishin was held responsible for the Soviet failure to
be first country to place a man on the moon.

Robert Gilruth was the NASA head of the Space Task Force and director of 25 manned space flights. Gilruth was the person who suggested to John F. Kennedy that the Americans take the bold step of reaching the Moon in an attempt to reclaim space superiority from the Soviets.

Targets of exploration

Image of the Sun from 7 June 1992 showing some sunspots

The Sun

While the Sun
will probably not be physically explored in the close future, one of
the reasons for going into space is to know more about the Sun. Once
above the atmosphere in particular and the Earth's magnetic field, this
gives access to the Solar wind and infrared and ultraviolet radiations
that cannot reach the surface of the Earth. The Sun generates most space weather,
which can affect power generation and transmission systems on Earth and
interfere with, and even damage, satellites and space probes.

Mercury

Mercury remains the least explored of the inner planets. As of May 2013, the Mariner 10 and MESSENGER missions have been the only missions that have made close observations of Mercury. MESSENGER
entered orbit around Mercury in March 2011, to further investigate the
observations made by Mariner 10 in 1975 (Munsell, 2006b).

A MESSENGER image from 18,000 km showing a region about 500 km across

A third mission to Mercury, scheduled to arrive in 2020, BepiColombo is to include two probes. BepiColombo is a joint mission between Japan and the European Space Agency.
MESSENGER and BepiColombo are intended to gather complementary data to
help scientists understand many of the mysteries discovered by Mariner
10's flybys.

Flights to other planets within the Solar System are accomplished at a
cost in energy, which is described by the net change in velocity of the
spacecraft, or delta-v.
Due to the relatively high delta-v to reach Mercury and its proximity
to the Sun, it is difficult to explore and orbits around it are rather
unstable.

Venus

Venus
was the first target of interplanetary flyby and lander missions and,
despite one of the most hostile surface environments in the solar
system, has had more landers sent to it (nearly all from the Soviet
Union) than any other planet in the solar system. The first successful
Venus flyby was the American Mariner 2
spacecraft, which flew past Venus in 1962. Mariner 2 has been followed
by several other flybys by multiple space agencies often as part of
missions using a Venus flyby to provide a gravitational assist en route to other celestial bodies. In 1967 Venera 4 became the first probe to enter and directly examine the atmosphere of Venus. In 1970 Venera 7
became the first successful lander to reach the surface of Venus and by
1985 it had been followed by eight additional successful Soviet Venus
landers which provided images and other direct surface data. Starting in
1975 with the Soviet orbiter Venera 9
some ten successful orbiter missions have been sent to Venus, including
later missions which were able to map the surface of Venus using radar to pierce the obscuring atmosphere.

Earth

Space exploration has been used as a tool to understand the Earth as a
celestial object in its own right. Orbital missions can provide data
for the Earth that can be difficult or impossible to obtain from a
purely ground-based point of reference.
For example, the existence of the Van Allen belts was unknown until their discovery by the United States' first artificial satellite, Explorer 1.
These belts contain radiation trapped by the Earth's magnetic fields,
which currently renders construction of habitable space stations above
1000 km impractical. Following this early unexpected discovery, a large
number of Earth observation satellites have been deployed specifically
to explore the Earth from a space based perspective. These satellites
have significantly contributed to the understanding of a variety of
earth based phenomena. For instance, the hole in the ozone layer
was found by an artificial satellite that was exploring Earth's
atmosphere, and satellites have allowed for the discovery of
archeological sites or geological formations that were difficult or
impossible to otherwise identify.

Earth's Moon

Earth's Moon
was the first celestial body to be the object of space exploration. It
holds the distinctions of being the first remote celestial object to be
flown by, orbited, and landed upon by spacecraft, and the only remote
celestial object ever to be visited by humans.
In 1959 the Soviets obtained the first images of the far side of the Moon, never previously visible to humans. The U.S. exploration of the Moon began with the Ranger 4 impactor in 1962. Starting in 1966 the Soviets successfully deployed a number of landers to the Moon which were able to obtain data directly from the Moon's surface; just four months later, Surveyor 1 marked the debut of a successful series of U.S. landers. The Soviet unmanned missions culminated in the Lunokhod program in the early '70s which included the first unmanned rovers and also successfully returned lunar soil samples to the Earth
for study. This marked the first (and to date the only) automated
return of extraterrestrial soil samples to the Earth. Unmanned
exploration of the Moon continues with various nations periodically
deploying lunar orbiters, and in 2008 the Indian Moon Impact Probe.

Manned exploration of the Moon began in 1968 with the Apollo 8 mission that successfully orbited the Moon, the first time any extraterrestrial object was orbited by humans. In 1969 the Apollo 11
mission marked the first time humans set foot upon another world.
Manned exploration of the Moon did not continue for long, however. The Apollo 17 mission in 1972 marked the most recent human visit there, and the next, Exploration Mission 2, is due to orbit the Moon in 2019. Robotic missions are still pursued vigorously.

Mars

The exploration of Mars
has been an important part of the space exploration programs of the
Soviet Union (later Russia), the United States, Europe, and Japan.
Dozens of robotic spacecraft, including orbiters, landers, and rovers,
have been launched toward Mars since the 1960s. These missions were
aimed at gathering data about current conditions and answering questions
about the history of Mars. The questions raised by the scientific
community are expected to not only give a better appreciation of the red
planet but also yield further insight into the past, and possible
future, of Earth.
The exploration of Mars has come at a considerable financial cost
with roughly two-thirds of all spacecraft destined for Mars failing
before completing their missions, with some failing before they even
began. Such a high failure rate can be attributed to the complexity and
large number of variables involved in an interplanetary journey, and has
led researchers to jokingly speak of The Great Galactic Ghoul[17] which subsists on a diet of Mars probes. This phenomenon is also informally known as the Mars Curse.[18] In contrast to overall high failure rates in the exploration of Mars, India has become the first country to achieve success of its maiden attempt. India's Mars Orbiter Mission (MOM)[19][20][21] is one of the least expensive interplanetary missions ever undertaken with an approximate total cost of 450 Crore (US$73 million).[22][23]

Phobos

The Russian space mission Fobos-Grunt, which launched on 9 November 2011 experienced a failure leaving it stranded in low Earth orbit.[24] It was to begin exploration of the Phobos
and Martian circumterrestrial orbit, and study whether the moons of
Mars, or at least Phobos, could be a "trans-shipment point" for
spaceships travelling to Mars.[25]

Jupiter

The exploration of Jupiter
has consisted solely of a number of automated NASA spacecraft visiting
the planet since 1973. A large majority of the missions have been
"flybys", in which detailed observations are taken without the probe
landing or entering orbit; the Galileo
spacecraft is the only one to have orbited the planet. As Jupiter is
believed to have only a relatively small rocky core and no real solid
surface, a landing mission is nearly impossible.

Reaching Jupiter from Earth requires a delta-v of 9.2 km/s,[26] which is comparable to the 9.7 km/s delta-v needed to reach low Earth orbit.[27] Fortunately, gravity assists through planetary flybys
can be used to reduce the energy required at launch to reach Jupiter,
albeit at the cost of a significantly longer flight duration.[26]

Jupiter has over 60 known moons, many of which have relatively little known information about them.

Saturn

Saturn has been explored only through unmanned spacecraft launched by NASA, including one mission (Cassini–Huygens) planned and executed in cooperation with other space agencies. These missions consist of flybys in 1979 by Pioneer 11, in 1980 by Voyager 1, in 1982 by Voyager 2
and an orbital mission by the Cassini spacecraft which entered orbit in
2004 and is expected to continue its mission well into 2012.
Saturn has at least 62 known moons,
although the exact number is debatable since Saturn's rings are made up
of vast numbers of independently orbiting objects of varying sizes. The
largest of the moons is Titan.
Titan holds the distinction of being the only moon in the solar system
with an atmosphere denser and thicker than that of the Earth. As a
result of the deployment from the Cassini spacecraft of the Huygens
probe and its successful landing on Titan, Titan also holds the
distinction of being the only moon (apart from Earth's own Moon) to be
successfully explored with a lander.

Uranus

The exploration of Uranus has been entirely through the Voyager 2 spacecraft, with no other visits currently planned. Given its axial tilt
of 97.77°, with its polar regions exposed to sunlight or darkness for
long periods, scientists were not sure what to expect at Uranus. The
closest approach to Uranus occurred on 24 January 1986. Voyager 2 studied the planet's unique atmosphere and magnetosphere. Voyager 2 also examined its ring system and the moons of Uranus including all five of the previously known moons, while discovering an additional ten previously unknown moons.
Images of Uranus proved to have a very uniform appearance, with no
evidence of the dramatic storms or atmospheric banding evident on
Jupiter and Saturn. Great effort was required to even identify a few
clouds in the images of the planet. The magnetosphere of Uranus,
however, proved to be completely unique and proved to be profoundly
affected by the planet's unusual axial tilt. In contrast to the bland
appearance of Uranus itself, striking images were obtained of the moons
of Uranus, including evidence that Miranda had been unusually geologically active.

Neptune

The exploration of Neptune began with the 25 August 1989 Voyager 2 flyby, the sole visit to the system as of 2014. The possibility of a Neptune Orbiter has been discussed, but no other missions have been given serious thought.
Although the extremely uniform appearance of Uranus during Voyager
2's visit in 1986 had led to expectations that Neptune would also have
few visible atmospheric phenomena, Voyager 2 found that Neptune had
obvious banding, visible clouds, auroras, and even a conspicuous anticyclone storm system
rivaled in size only by Jupiter's small Spot. Neptune also proved to
have the fastest winds of any planet in the solar system, measured as
high as 2,100 km/h.[28]
Voyager 2 also examined Neptune's ring and moon system. It discovered
900 complete rings and additional partial ring "arcs" around Neptune. In
addition to examining Neptune's three previously known moons, Voyager 2
also discovered five previously unknown moons, one of which, Proteus, proved to be the last largest moon in the system. Data from Voyager further reinforced the view that Neptune's largest moon, Triton, is a captured Kuiper belt object.[29]

Other objects in the Solar system

Pluto

Pluto and Charon (1994)

The dwarf planet Pluto (considered a planet until the IAU redefined "planet" in October 2006[30])
presents significant challenges for spacecraft because of its great
distance from Earth (requiring high velocity for reasonable trip times)
and small mass (making capture into orbit very difficult at present). Voyager 1 could have visited Pluto, but controllers opted instead for a close flyby of Saturn's moon Titan, resulting in a trajectory incompatible with a Pluto flyby. Voyager 2 never had a plausible trajectory for reaching Pluto.[31]

Pluto continues to be of great interest, despite its reclassification
as the lead and nearest member of a new and growing class of distant
icy bodies of intermediate size, in mass between the remaining eight
planets and the small rocky objects historically termed asteroids (and
also the first member of the important subclass, defined by orbit and
known as "Plutinos"). After an intense political battle, a mission to Pluto dubbed New Horizons was granted funding from the US government in 2003.[32]New Horizons was launched successfully on 19 January 2006. In early 2007 the craft made use of a gravity assist from Jupiter.
Its closest approach to Pluto will be on 14 July 2015; scientific
observations of Pluto will begin five months prior to closest approach
and will continue for at least a month after the encounter.

Asteroids and comets

Until the advent of space travel, objects in the asteroid belt
were merely pinpricks of light in even the largest telescopes, their
shapes and terrain remaining a mystery. Several asteroids have now been
visited by probes, the first of which was Galileo, which flew past two: 951 Gaspra in 1991, followed by 243 Ida
in 1993. Both of these lay near enough to Galileo's planned trajectory
to Jupiter that they could be visited at acceptable cost. The first
landing on an asteroid was performed by the NEAR Shoemaker probe in 2000, following an orbital survey of the object. The dwarf planet Ceres and the asteroid 4 Vesta, two of the three largest asteroids, are targets of NASA's Dawn mission, launched in 2007.

Hayabusa was an unmanned spacecraft developed by the Japan Aerospace Exploration Agency to return a sample of material from a small near-Earth asteroid named 25143 Itokawa
to Earth for further analysis. Hayabusa was launched on 9 May 2003 and
rendezvoused with Itokawa in mid-September 2005. After arriving at
Itokawa, Hayabusa studied the asteroid's shape, spin, topography,
colour, composition, density, and history. In November 2005, it landed
on the asteroid to collect samples. The spacecraft returned to Earth on
13 June 2010.

Deep space exploration

Future of space exploration

Concept art for a NASA Vision mission

In the 2000s, several plans for space exploration were announced;
both government entities and the private sector have space exploration
objectives. China has announced plans to have a 60-ton multi-module
space station in orbit by 2020.

The NASA Authorization Act of 2010
provided a re-prioritized list of objectives for the American space
program, as well as funding for the first priorities. NASA proposes to
move forward with the development of the Space Launch System (SLS), which will be designed to carry the Orion Multi-Purpose Crew Vehicle,
as well as important cargo, equipment, and science experiments to
Earth's orbit and destinations beyond. Additionally, the SLS will serve
as a back up for commercial and international partner transportation
services to the International Space Station. The SLS rocket will
incorporate technological investments from the Space Shuttle program and
the Constellation program in order to take advantage of proven hardware
and reduce development and operations costs. The first developmental
flight is targeted for the end of 2017.[33]

AI in Space Exploration

The idea of using high level automated systems for space missions has
become a desirable goal to space agencies all around the world. Such
systems are believed to yield benefits such as lower cost, less human
oversight, and ability to explore deeper in space which is usually
restricted by long communications with human controllers.[34]

Autonomous System

Being able to sense the world and their state, make decisions, and carry them out on their own

Can interpret the given goal as a list of actions to take

Fail flexibly

Benefits

Autonomed technologies would be able to perform beyond predetermined
actions. It would analyze all possible states and events happening
around them and come up with a safe response. In addition, such
technologies can reduce launch cost and ground involvement. Performance
would increase as well. Autonomy would be able to quickly respond upon
encountering an unforeseen event, especially in deep space exploration
where communication back to Earth would take too long.[34]

NASA’s Autonomous Science Experiment

NASA began its autonomous science experiment (ASE) on the Earth
Observing 1 (EO-1) which is NASA’s first satellite in the new millennium
program Earth observing series launched on 21 November 2000. The
autonomy of ASE is capable of on-board science analysis, replanning,
robust execution, and later the addition of model-based diagnostic.
Images obtained by the EO-1 are analyzed on-board and downlinked when a
change or an interesting event occur. The ASE software has successfully
provided over 10,000 science images.[34]

Rationales

Astronaut Buzz Aldrin, had a personal Communion service when he first arrived on the surface of the Moon.

The research that is conducted by national space exploration agencies, such as NASA and Roscosmos,
is one of the reasons supporters cite to justify government expenses.
Economic analyses of the NASA programs often showed ongoing economic
benefits (such as NASA spin-offs), generating many times the revenue of the cost of the program.[35]
It is also argued that space exploration would lead to the extraction
of resources on other planets and especially asteroids, which contain
billions of dollars worth of minerals and metals. The revenue generated
from such expeditions could generate a lot of revenue.[36] As well, it has been argued that space exploration programs help inspire youth to study in science and engineering.[37]

Another claim is that space exploration is a necessity to mankind and that staying on Earth will lead to extinction. Some of the reasons are lack of natural resources, comets, nuclear war, and worldwide epidemic. Stephen Hawking,
renowned British theoretical physicist, said that "I don't think the
human race will survive the next thousand years, unless we spread into
space. There are too many accidents that can befall life on a single
planet. But I'm an optimist. We will reach out to the stars."[38]

NASA has produced a series of public service announcement videos supporting the concept of space exploration.[39]

Overall, the public remains largely supportive of both manned and unmanned space exploration. According to an Associated Press
Poll conducted in July 2003, 71% of U.S. citizens agreed with the
statement that the space program is "a good investment", compared to 21%
who did not.[40]

Arthur C. Clarke (1950) presented a summary of motivations for the human exploration of space in his non-fiction semi-technical monograph Interplanetary Flight.[41]
He argued that humanity's choice is essentially between expansion off
the Earth into space, versus cultural (and eventually biological)
stagnation and death.

A spaceflight typically begins with a rocket launch, which provides the initial thrust to overcome the force of gravity
and propels the spacecraft from the surface of the Earth. Once in
space, the motion of a spacecraft—both when unpropelled and when under
propulsion—is covered by the area of study called astrodynamics. Some spacecraft remain in space indefinitely, some disintegrate during atmospheric reentry, and others reach a planetary or lunar surface for landing or impact.

Satellites

Satellites are used for a large number of purposes. Common types
include military (spy) and civilian Earth observation satellites,
communication satellites, navigation satellites, weather satellites, and
research satellites. Space stations and human spacecraft in orbit are also satellites.

Alien life

Astrobiology is the interdisciplinary study of life in the universe, combining aspects of astronomy, biology and geology.[42] It is focused primarily on the study of the origin, distribution and evolution of life. It is also known as exobiology (from Greek: έξω, exo, "outside").[43][44][45]
The term "Xenobiology" has been used as well, but this is technically
incorrect because its terminology means "biology of the foreigners".[46] Astrobiologists must also consider the possibility of life that is chemically entirely distinct from any life found on earth.[47] In the Solar System some of the prime locations for current or past astrobiology are on Enceladus, Europa, Mars, and Titan.[48]

Living in space

Space colonization, also called space settlement and space
humanization, would be the permanent autonomous (self-sufficient) human habitation of locations outside Earth, especially of natural satellites or planets such as the Moon or Mars, using significant amounts of in-situ resource utilization.

To date, the longest human occupation of space is the International Space Station which has been in continuous use for 14 years, 69 days. Valeri Polyakov's record single spaceflight of almost 438 days aboard the Mir
space station has not been surpassed. Long-term stays in space reveal
issues with bone and muscle loss in low gravity, immune system
suppression, and radiation exposure.

Many past and current concepts for the continued exploration and
colonization of space focus on a return to the Moon as a "stepping
stone" to the other planets, especially Mars. At the end of 2006 NASA
announced they were planning to build a permanent Moon base with
continual presence by 2024.[49]

About Me

My formal training is in chemistry. I also read a great deal of physics and biology. In fact I very much enjoy reading in general, mostly science, but also some fiction and history. I also enjoy computer programming and writing. I like hiking and exploring nature. I also enjoy people; not too much in social settings, but one on one; also, people with interesting or "off-beat" minds draw me to them. I also have some interest in Buddhism.

These days I get a lot more information from the internet, primarily through Wiki. Some television, e. g., documentaries, PBS shows like "Nova" and "Nature".

My favorite science writers are Jacob Bronowski ("The Ascent of Man") and Richard Dawkins (his "The Blind Watchmaker" is right up there up Ascent). I also have a favorite writer on Buddhism, Pema Chodron. Favorite films are "Annie Hall" (by Woody Allen), "The Maltese Falcon", "One Flew Over The Cuckoo's Nest", "As Good As It Gets", "Conspiracy Theory", Monty Python's "Search For The Holy Grail" and "Life of Brian", and a few others which I can't think about at the moment.

I love a number of classical works (Beethoven's "Pastoral", "Afternoon Of A Fawn" and "Clair De Lune" by Debussey , Pachelbel's "Canon" come to mind. My favorite piece is probably Gershwin's "Rhapsody in Blue". But I also enjoy a great deal in modern music, including many jazz pieces, folk songs by people like Dylan, Simon and Garfunkel, a hodgepodge of pieces by Crosby, Stills, and Nash, Niel Young, and practically everything the Beatles wrote.

My life over the last few years has been in some disarray, but I am finally "getting it together.". As I am very much into the sciences and writing, I would like to move more in this direction. I also enjoy teaching. As for my political leanings, most people would probably describe as basically liberal, though not extremely so. My religious leanings are to the absolutely none: I've alluded to my interest in Buddhism, but again this is not any supernatural or scientifically untested aspect of it but in the way it provides a powerful philosophy and set of practical, day to day methods of dealing with myself and the other human beings.